infra: replace BUILDROOT_DL_DIR with BR2_DL_DIR.

To make the naming consistent (all user-visible options should be
prefixed BR2_).

An entry is added to Makefile.legacy to warn users who have set
BUILDROOT_DL_DIR but not BR2_DL_DIR.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>